1. Field of the Invention
The present invention relates to a thermal head which is used for thermal recording such as facsimiles, printers and the like.
2. Description of the Related Art
As shown in FIG. 4, a conventional thermal head has been manufactured in a manner that a glaze layer is provided as a heat storage layer on an insulating substrate such as a ceramic substrate and the like, a heat resistor material such as a Ta system material, a silicide material, a Nixe2x80x94Cr alloy and the like and an electrode material such as Al, Crxe2x80x94Cu, Au, and the like are deposited by using a sputtering method, a vapor deposition method or the like, a heat resister, wiring electrodes of an individual electrode 1 and a common electrode 2 are formed by patterning through a photolithographic process, and then a protective film such as SiO2, Ta2O5, SiAlON, Si3N4, SiC, and the like is deposited for preventing oxidation and wear of the heat resistor by using a sputtering method, an ion plating method, and a CVD method.
In recent years, a thermal printer having a thermal head has been used for small information terminal devices represented by small handy terminals which are small in size, light weight, and maintenance free.
For the small information terminal devices, electric power saving and high-speed printing are required because the small information terminal devices are driven by a battery. However, while at the present time a driving voltage of 5V is mainly used for the driving voltage of the thermal head, it is though that a driving voltage of 3V will be required in future. Further, reduction of the size of the thermal head and miniaturization of each wiring pattern have been proceeded with a decrease in the driving voltage. For this reason, the driving voltage of the thermal head is reduced, resistance is decreased by further miniaturization, and current consumption is increased, so that wiring resistance of a common electrode can not be neglected, even though the wiring resistance of the common electrode has been neglected. When the wiring resistance of the common electrode becomes higher, a so-called voltage drop occurs which causes a phenomenon of unevenness of printing density such that the printing density becomes low with a dot distant from a power supply in case of printing when voltage is applied to entire dots.
In order to solve these problems, it is necessary to enlarge a cross-sectional area (widthxc3x97thickness) of the electrode, which causes the wiring resistance to be reduced. It is thought as one of methods for reducing the wiring resistance that the width of the electrode is enlarged. However, the enlargement of the width is limited because of limitations of space for wiring density in the individual electrode, and the width of the common electrode is also limited because of limitations of the size of the entire thermal head.
Another method for reducing the wiring resistance involves increasing the thickness of the electrode. However, increasing the thickness of the individual electrode in the heat resistor portion results in deterioration of the contact with the thermal recording paper and a decrease in printing efficiency. Production efficiency of forming electrode also becomes worse with an increase in the thickness of the electrode, thereby resulting in an increase in cost.
Generally, several methods for solving the above described problems have been known. First, a so-called wiring resistance correction in which the width and length of the heat resistor and the individual electrode are varied in each dot in order to adjust the resistance according to a distribution of the wiring resistance is known. Second, a so-called conductor printing reinforcement in which a conductive paste such as silver, gold, and the like is formed on an upper layer or a lower layer of the common electrode by printing and burning in order to reinforce the wiring resistance of the common electrode where current concentration is large is also known. Third, a method in which a buildup circuit such as FPC and the like is connected to the common electrode is also known.
However, in the wiring resistance correction for solving the unevenness of the printing density due to the voltage drop, there are drawbacks such that the entire printing density is reduced because adjustment is set to a higher value side of the wiring resistance even if the unevenness of the printing density can be eliminated, or on the contrary the unevenness of the printing density is enlarged because a correction rate is different depending on electric current consumption by a printing pattern or a printing ratio.
In the conductor printing reinforcement, there are drawbacks such that accuracy of printing position becomes a problem with progress in down-sizing and miniaturization, which can not sufficiently correspond to manufacturing of the thermal head, reliability is decreased by reduction of coverage of the protective film because the surface roughness is large due to the particle size of the conductive paste, and sometimes contact resistance between the common electrode of a thin film and the conductor buildup layer of a thick film is generated by burning condition of the conductive paste, which results sufficient effect of the conductor printing reinforcement not to be obtained.
In case that the buildup electrode is formed by using a thin film in order to solve the problems, though it is thought that the buildup electrode is formed by laminating a plurality of common electrodes, because a cross-sectional shape of a peripheral portion of the common electrode is almost a vertical shape by etching and the like, coverage of the common electrode of the upper layer is not good, which causes a problem such that a lack of electric continuity increases the contact resistance between the common electrode and the individual electrode in the step portion of the periphery of the common electrode.
In view of the foregoing, it is an object of the invention to obtain a thermal head which eliminates the unevenness of the printing density in case of printing with a low-voltage driving printer, secures the contact resistance between the common electrode and the individual electrode, and performs the higher efficiency of heating for printing in a manner that the pattern accuracy is improved corresponding to the further miniaturization and the wiring resistance is restrained in a low value when the common electrode is formed.
In order to solve the above-described problems, according to the invention, a thermal head includes at least a heat resistor, an individual electrode for supplying electric power to the heat resistor, a common electrode connected to the individual electrode, and a protective film covering the heat resistor and the individual electrode of the periphery of the heater resistor on an insulating substrate, a peripheral portion of the buildup electrode has a tapered shape and a buildup electrode is formed to be covered by the common electrode.
In the thermal head having the above-described structure, since the common electrode is formed to cover the buildup electrode and the peripheral portion of the buildup electrode of a lower layer has the tapered shape, and the wiring resistance is restrained in a low value and electric continuity is secured in a step portion of the periphery of the common electrode, so that contact resistance between the common electrode and the individual electrode can be secured. As a result, the unevenness of the printing density is eliminated because the influence of the voltage drop is eliminated in case of printing by the low-voltage driving printer, and the higher efficiency of heating for printing can be performed, so that printing quality is improved.